Scientists used next-generation sequencing to analyze deep-sea corals following the Deepwater Horizon incident. They found that corals covered in a flocculent material had a diminished capacity to maintain crucial cellular mechanisms, likely because the corals had high metabolic demands associated with stress responses and repair mechanisms. There were 1,439 differentially expressed genes (≥ two-fold) among floc-exposed colonies vs controls; genes involved in oxidative stress, immunity, wound repair, tissue regeneration, and metabolism of xenobiotics were significantly more active in those corals covered with flocculant. The results provide evidence of genome‐wide cellular stress responses of affected corals to oil and dispersant exposure.

Following Deepwater Horizon, octocorals near the incident showed visible signs of distress as recorded by high-definition imagery. Coral stress responses, like many other organisms, can be revealed by examining changes in gene expression. As a graduate student, study author Danielle DeLeo (now a post-doctoral researcher at Florida International University) and other ECOGIG consortium members began investigating gene expression changes to help explain these impacts.

DeLeo described their method, “We used an RNA sequencing/transcriptomics approach to investigate the extent of Deepwater Horizon impacts by collecting, assembling, and analyzing a de novo transcriptome for the octocoral Paramuricea biscaya, the most common deep-sea coral species impacted by the spill (Fisher et al. 2014, PNAS). The corals were sampled and preserved on the seafloor (in situ) shortly after the spill when oil and dispersant were still present in the deep sea (as floc). Corals of this genus have also been subjected to numerous anthropogenic disturbances in the Mediterranean Sea.”

Previous studies found evidence that these corals were affected by oil and dispersant associated with Deepwater Horizon. Analysis of hopanoid petroleum biomarkers isolated from the floc indicated that the material contained oil associated with the Macondo well (White et al. 2012 PNAS). Analysis of floc and nearby sediment using liquid chromatography with tandem mass spectrometry indicated the presence of DOSS, a surfactant in Corexit (White et al. 2014 Environmental Science and Technology). Scientists developed and validated a high-resolution mass spectrometry method that reduced some of the challenges in detecting small quantities of DOSS, and their analyses confirmed the presence of DOSS in deep-sea sediments near the spill site (Perkins et al. 2017 Analytical and Bioanalytical Chemistry).

The team’s integrative approach included field sampling with remote- and human-operated vehicles, molecular lab work to extract high-quality RNA for sequencing, and analyses of large transcriptomic datasets using bioinformatics. This high-throughput sequencing approach was conventionally reserved for model organisms (or close relatives) with sequenced genomes, but has recently been applied to non-model organisms as advancements in bioinformatic tools allow for unguided, de novo assembly.

“Our findings suggest that the corals were investing a lot of energy to cope with the stress of the floc exposure and repair cellular damage. It also implies that floc exposure was toxic at the cellular level and the portions of the impacted corals sampled and sequenced were nearing irreparable damage,” explained DeLeo.

The authors noted that their findings suggest that elevated stress response proteins may have conferred some resistance to cells and tissues that were not in primary contact with the floc and directly exposed to suspected crude oil/dispersant constituents. The elevated stress response proteins in conjunction with hyper-melanization of damaged regions of the colony may have established a barrier around compromised tissues, enabling partial coral colony survival.

The team’s effort resulted in the first published transcriptome assembly for a deep-sea octocoral, “This study provides a unique perspective to the Deepwater Horizon damage assessment process, and this novel dataset will be useful for the development of biomarkers for use in future oil spill response and monitoring efforts of deep-sea invertebrates,” said DeLeo. “More broadly, it will contribute to the fundamental understanding of octocoral stress responses, whether it be from oil spills, or other anthropogenic and natural stressors.”

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.